Known circuit arrangements for operating a discharge lamp include a resonant circuit, having a capacitive element which is connected in parallel with the discharge lamp and an inductive element which is in series upstream of the lamp and the capacitive element and downstream of a switching point between two switches. The switches, which typically take the form of MOSFETs, serve to apply current to the resonant circuit. Suitable means cause the two switches to be closed and opened again in a mutually alternating sequence, at predetermined switching frequency. Typically, the means for causing this include an application-specific integrated circuit (ASIC), which connects potential outputs with the control inputs of MOSFETs.
The purpose of the resonant circuit is to ensure that an ignition voltage is provided through the electrodes of the lamp, in other words in parallel with the capacitive element of the resonant circuit. To this end, a square voltage is applied over a half-bridge circuit to bring about oscillation or near resonance. So that the ignition voltage is set in defined manner, adjustment to give a predetermined amperage of the alternating current is performed by varying the frequency of the square voltage. To this end, means are provided in the circuit arrangement for measuring the amperage of a current flowing through one of the switches, and suitable means for establishing the switching frequency establish the latter during ignition as a function of the measured amperage.
Before the discharge lamp is ignited, the electrodes, which are typically in the form of coils, have to be pre-heated. Pre-heating is performed by providing heat from an ohmic loss by sending a current through the electrodes. This is also carried out in the circuit arrangement by triggering the switches and hence applying alternating current to the resonant circuit. In this case, however, the frequency is different from that used when the lamp is ignited.
In circuit arrangements that have been used hitherto, the frequency of the alternating current during pre-heating was established in advance by setting it to values above the resonant frequency of the resonant circuit.
The electrical parameters of electronic components may vary from one individual component to the next even if a nominal value is desirable per se. If the frequency during pre-heating is established in advance, as was the case hitherto, the pre-heating current that is set is highly sensitive to the parameters of the electronic components, in particular the capacitive element and the inductive element of the resonant circuit. It is then possible for a circuit arrangement to be rejected during production as not functioning adequately, even if all the components are individually functional per se and only deviations in the parameters are present in the components.
Adjustment of the pre-heating current is known in a number of circuit arrangements.